It was a Monday.
December 12 can fall on any day of the week.
September 12 1988 was a Monday.
March 12 1988 was a Saturday.
May 12, 1988 fell on a Thursday.
December 12, 1990 was a Wednesday.
December 12 1947 was a Friday.
December 12 1958 was a Friday.
December 12 2010 was a Sunday.
Monday
Saturday
December 12, 1972 fell on a Tuesday.
December 12, 1765 fell on a Thursday.